It's the Class 321's they are losing, which have been replaced by the new Desiro build.
This order surprises me slightly. With the imminent cascade of the LM321 fleet, and the arrival by 2012 of a fleet of Thameslink trains, (let along Crossrail stock a few years later), I would have thought there would be an excessive number of emus by the time the Olympics has been and gone. The 319 fleet will only be 25 years old, which for electric stock is well short of life expiry.
Of course, if they electrify the GWML▸ and MML» that would soak a few up.
But they won't. This is the disjointed fragmented British railway network we're talking about, so we'll end up with a situation where they are scrapping electric trains with many years of life left in them, while the majority of the network suffers from a desperate shortage of diesels.